Referring to FIG. 1, an image sensor according to the prior art is shown. FIG. 1 provides a schematic sectional view of a first pixel 1, adjacent to a second pixel 2 of a CMOS image sensor of a type with front side illumination (FSI). The two pixels have analogous structures. The two pixels may, for example, form part of a Bayer pattern, which is well known to the skilled artisan.
The first pixel 1 is produced on a semiconductor substrate 10, within which is a photoreceptor, e.g., a photodiode. The substrate 10 is surmounted by an interconnection part or portion 11, commonly referred to as the BEOL (Back End Of Line). An antireflection layer 4 may be produced between the photodiode and the interconnection portion to ensure good transmission of the light rays.
The interconnection portion 11 includes various metallization levels M1, M2 and vias shrouded in one or more dielectric materials. Color filters 12 and 22 are situated on the interconnection portion 11, facing the photodiodes. It should be noted that the metallization levels M1 and M2 are produced in such a way that they are not situated in the region between the optical filter and the photodiode. Only a dielectric region covers the antireflection layer 4. The pixel is surmounted by a collimation lens 13, making it possible to optimize the collection of the light rays at the level of the photodiode.
The color filter 12 of the first pixel 1 may, for example, be configured to allow the wavelength corresponding to the color red to pass, and the filter 22 of the second pixel 2 may be configured to allow the wavelength corresponding to the color green to pass.
During operation, the photons absorbed by the photodiode cause the generation of charge carriers, which create an electric current in the photodiode. The mutual proximity of the pixels may give rise to a particularly significant crosstalk phenomena in image sensors having reduced or relatively small dimensions. A crosstalk arises when an optical signal 3 arriving, for example, at the first pixel 1 is not totally collected by the corresponding photodiode 10, thus degrading the performance of the sensor, especially the color rendition.
An optical crosstalk is when the photons passing through a filter reach the photodiode of an adjacent pixel. For example, the optical signal 32, after having been filtered by the optical filter 22 of the second pixel 2 (optical signal 31), reaches the photodiode 10 of the first pixel 1 instead of reaching the photodiode 20 of the second pixel 2.
A spectral crosstalk is when the optical filter is not selective enough and allows through wavelengths for which it is not configured. For example, a luminous signal, after passing within the green filter may comprise wavelengths below or above that of green.
The crosstalk may also be of electrical origin when the electrons generated by the photodiode of a pixel disperse in an adjacent pixel. This crosstalk of electrical origin is not the subject of the present patent application.
In FIG. 2 the curves of quantum efficiency of a set of pixels forming a Bayer pattern are illustrated. The three curves B, G, R correspond respectively to the quantum efficiency of the blue, green and red pixels of the sensor. The part G1 of the curve G shows that the green pixel detects a significant share of signals whose wavelength corresponds to the color blue. In an analogous manner, the part R1 of the curve R shows that the red pixel detects a significant share of signals, including the wavelength corresponding to the color green. These spurious detections attest to the crosstalk phenomena explained above.